Digital cable television providers broadcast television programs to subscribers over channels that are six MHz wide in North America and eight MHz wide in Europe. Before being broadcast, the data of a television program is compressed and modulated, typically using 64 or 256 quadrature amplitude modulation (QAM).
Using 256 QAM, a six MHz wide channel can provide an approximate transmission rate of 39 Mbps under normal noise conditions, which is enough capacity to support the transmission of several standard-definition television (SDTV) programs and/or high-definition television (HDTV) programs. With the introduction of improved video compression technologies, such as High Efficiency Video Encoding (HEVC) or H.265, the approximate 39 Mbps channel capacity can even support the transmission of one ultra high-definition television (UHDTV) program. For example, a 4K UHDTV program at 60 frames/sec and 10 bits/pixel compressed using HEVC requires an average data rate of about 25 Mbps, which can fit within the 39 Mbps capacity of the six MHz wide channel using 256 QAM.
One issue encountered using this basic transmission scheme is that, after packing one or more compressed and modulated broadcast television programs into a channel, there is often left over channel capacity that is wasted because it is too small to accommodate the transmission of another compressed and modulated broadcast television program. As UHDTV formats become ubiquitous, there is an enormous amount of additional video bits that need to be compressed and transported, making increased utilization of this left over channel capacity desirable.
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